Ultrasonic Signal Processing for Structural Damage Detection and Quantification
The operation of our society depends heavily on infrastructure systems. To prevent failures and to reduce costs of maintenance, structural health monitoring (SHM) systems have been implemented on an increasing number of infrastructure systems. SHM systems have the potential to give reliable prediction of structural deterioration with less human safety risk and labor costs, and without interruption of normal operations. In the field of SHM, many techniques have been proposed in recent decades. Among these techniques, ultrasonic testing has been widely used for damage characterization in structures and materials. However, there remain many challenges in real-world SHM applications. For example, temperature variations can cause a significant decrease in performance of ultrasonic testing. Although there exist some temperature compensation techniques to improve the performance of ultrasonic testing under temperature variations, these techniques have their own limitations. This dissertation will focus on novel ultrasonic signal processing techniques for damage detection, quantification and temperature compensation. In Chapter 2, I will propose a modified optimal signal stretching (OSS) method and an singular value decomposition (SVD) method to solve the temperature compensation problem, where the OSS method (in its original form) failed to perform well for damage detection. In Chapter 3, I will study the statistical orthogonal relationship between temperature-induced and damage-induced ultrasonic change signals. The orthogonal relationship can be used to explain why SVD performs well under varying temperature conditions and why it also has the potential (under some conditions) to be directly used for damage detection and quantification. In Chapter 4, I viii will study the ultrasonic time-of-flight diffraction technique, which is used to quantify wall thickness loss of thick-walled aluminum tubes, because the conventional pulse-echo method did not perform well in my target application. In Chapter 5, I will propose a novel ultrasonic passband technique to quantify the alkali-silica reaction (ASR) caused cracking damage in concrete structures. This technique is based on the ultrasonic wave filtering effects of cracks in concrete. With the progress of ASR caused cracking damage in concrete, more high frequency components of ultrasonic waves are filtered out than low frequency components. The research work in this dissertation has the potential to help advance ultrasonic SHM techniques, to improve the real-world performance of ultrasonic SHM, to prevent failures of infrastructure systems, and to reduce the costs of maintenance if the proposed ultrasonic techniques can be implemented in real infrastructure systems in the future. However, some future work still needs to be done in order to implement the techniques studied in this dissertation in real-world applications.